Electromechanical transducer



Aug 7, 956 ww. SHANNON ET AL 2,758,288

ELECTROMECHANICAL TRANSDUCER 7 Filed Jan. 29, 1952 s Sheets-Sheet 1 PIC-3.3

INVENTORS WILLIAM W. SHAN NON HERBERT C ROTERS BY L' ,77lmm Mm ATTORNEYS Aug. 7, 1955 W. W. SHANNON ETAL ELECTROMECHANICAL TRANSDUCER 5 sneaks-sheet 2 Filed Jan. 29, 1952 mdE mdE

comtwoa 9.39.5

INVENTORS WILLIAM W. SHANNON HER BERT C. ROTERS Yul ATTORNEYS 7, 1956 w. w. SHANNON ET AL 2,758,288

ELECTROMECHANICAL TRANSDUCER 5 Sheets-Sheet 3 Filed Jan. 29, 1952 INVENTORS WILLIAM W. SHANNON BY HERBERT G. ROTERS 714m film 1.0104

ATTORNEYS United States Patent ELECTROMECHANTCAL TRANSDUCER William Wilson Shannon, Glen Wood Landing, and Herbert C. Roters, Kew Gardens, N. Y., assignors to Servomechanisms, Inc., a corporation of New York Application January 29, 1952, Serial No. 268,798

2 Claims. (Cl. 336-30) This invention relates to electro-mechanical transducers and more particularly to such transducers of the type suitable for deriving an electrical signal representative of a mechanical effect, for example of a mechanical move ment indicative of ambient pressure, and will be particularly described in such an embodiment. The term electromechanical transducer is used herein to refer to an apparatus comprising a magnetic core structure provided with a plurality of windings at least some of which are excited by an alternating current and having a magnetic armature for varying the distribution of the core flux through the several windings and movable in accordance with an input displacement or mechanical effect, an output signal being derived in accordance with the variation of the distribution of the core fluxes.

it is frequently desirable to derive and transmit to a re mote point an electrical signal representative of a displacement or like mechanical effect, as in telemetering and re mote control systems for developing indications or control effects representative of pressure, temperature, stress, etc.

For maximum accuracy and reproducibility, it is de sirable that an electro-mechanical transducer have the following characteristics: stability, that is, a response characteristic which is reproducible regardless of aging and regardless of cyclic variations of stress, temperature, etc., to which the device may be subjected; a high sensitivity or scale factor, that is, a large amplitude electrical signal output for a given mechanical input; facility of adjustment of the Zero input reading and the calibration scale of the device without adjustment of the armature, since provisions for adjusting the armature tend to limit the mechanical rigidity and stability of the device; symrnetry of the armature relative to the core structure, which makes the device relatively insensitive to spurious displacements of the armature; an armature position for Zero signal output at one extreme position on one side of the position of mechanical symmetry, which increases the range of linear response in those cases in which the input effect varies in one sense from a given reference value; a high degree of linearity of response with freedom of distortion over the normal operating range, which distortion is usually due to mechanical or electrical asymmetry, non-linear operating or reacting forces on the armature, a shift in phase between the exciting signal and the output signal, or to a combination of these factors. Electro-mechanical transducers heretofore proposed have lacked some or all of the foregoing advantageous characteristics.

It is an object of the present invention, therefore, to provide a new and improved electro-mechanical transducer which embodies one or more of the above-mentioned desirable characteristics.

It is another object of the invention to provide a new and improved electro-mechanical transducer which is particularly characterized by extreme linearity of response and ease of calibration.

In accordance with the invention, there is provided an electro-mechanical transducer comprising a closed magnetic core structure having two opposed pairs of inwardly extending poles and a winding disposed on each of the poles, such windings being connected to form a bridge circuit. Alternating current input terminals are connected to one pair of diagonal terminals of the bridge, signal output terminals are connected to the-other pair of diagonal terminals of the bridge, and a magnetic arma-i ture is disposed between the opposed pairs of poles and is movable in response to a mechanical effect tobe re resented.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection" with the accompanying drawing, while its scopewill pointed out in the appended claims.

Referring now to the drawings- Fig. 1a is a schematic representation of an electromechanical transducer embodying the invention;

Fig. 1b is a cross-sectional view of the apparatus of Fig. 1a;

Fig. 2 is a simplified equivalent electrical circuit diagram of the apparatus of Fig. la; V

Fig. 3 is a vector diagram to aid in the explanation of the invention;

Fig. 4 is a fragmentary enlarged view of the movable magnetic armature of the apparatus of Fig. 1a;

Fig. 5 is a graph representing certain operating charac teristics of the apparatus of the invention;

Fig. 6 is an exploded isometric view of a device incorporating the transducer of the invention and illustrating a preferred application thereof; and

Figs. 7a-7c inclusive, 8a-8c inclusive, and 9a-9e inelusive, are schematic representations of certain modified forms of the electromechanical transducer of the invention.

Referring now more particularly to Figs. 1a and 1b of the drawings, there is schematically represented an electro-mechanical transducer embodying the invention and comprising a circular closed magnetic core structure 9 having two pairs of opposed parallel inwardly extending poles 10, 11 and 12, 13 having parallel opposed pole faces 10a, 11a and 12a, 13a, respectively. Individually disposed on each of the pairs of poles 10, 11 and 12, 13 are windings or coils 14, 15 and 16, 17, respectively, these windings being suitably insulated from the magnetic core structure, as by a series of spools 18 of insulation material.

The windings 1417 inclusive are connected to form a conventional bridge circuit with pairs of diagonal-tel resistance to reactance, be equal. A pair of alternating current input terminals 21 is connected to the diagonal terminals 19 of the bridge, while the other diagonal terminals 20 are connected to signal-output terminals 27, 27 of the apparatus to which is connected an indicating device, such as a voltmeter 22 which may be calibrated to read directly in terms of the mechanical efiect to which the apparatus is responsive, as describedhereim. after.

In determining equal values of: impedance, it is important that both the absolute values of impedance and the phase angles, that is, the ratio of The transducer ofthe-invention also includes a magnetictarmaturc 23 pivoted abouttheaxisv 2.4, .which is. the axis of symmetry of the core structure 9. The armature 23 is in the form of a thin elongated vane which is dispose'd'ibetween' the-opposedpairsof poles 1-0, l'l an'd 12, 13-=and1 is movable by any conventional: actuating I mechanismz in 1 response to asm'echanical effect toxbe. represented. The vane 2-3 iszill'ustrated in one'extreme positionof its movement, .while -thedotted line representations 23a and ZSb'indicate its neutral position and its other extreme position, "respectively. These extreme positions of movement "of ;the armature .23 has been considerably exaggenatedtfor'the 'sake of clarity. In an actual design the rangewof' movement-pf i the armature 23 is made a very small fractiomo'f therspacing of the poles between which it is disposed.

in-order torcornpensate for variations in the response characteristic ofithettransducerrof, the invention withvariationsainambient-temperature, :it alsoincludes a pairof magneticsshuntsi25 sand .26, each extending between a pole face of at least one pole of.each ofthe-pairs of poles 10, 11 and 12, -13.:;andthe body of :the core-structure'flaand"havingiaitemperature coefficient of permeability of a senserand:valuetproportioned substantially to cornpensate .ifor rthe 3 temperature variations .of .the response characteristic. In one embodiment of the invention, the magnetic s'shunts r25,..26..-are :eachin the form of-an .L, with one arm of shunt 25 overlying the=pole:face a and onerarm :ofrshunt 22.6..overlying the pole face 13a. The other arm of each shunt is intimately securedstoa surfaceof the GOrerstructureB.

It is noted thattthe windings Mend '16fiI'CtCOI1I16CtCd in series acrossthe inputterminals 219, while the windings 15 and 17 formaa' similar 1-Series circuit. Therelative polarityof connection of the windings is .such that the windings and r15..coopcrate to send flux in one direction through the poleslfi-andll, as: indicated in-Fig. 1a by the fiuxesenand z,-whilethe windings 16 and.17 similarly cooperate to sendflux in the same direction through the poles 12' and'13', as indicated by fluxes. and.4. The pole face are normal to the axes of the poles so as to form a rectangular air gap between the poles of each pair which is symmetrical about an axis b, b normal to the axesof 'thepoles and passing through the axis of -symmetry 24 of the corezstructure.

1 Reference iswnow made to Figs. 2, 3 and 4 of the drawingsforsan explanation of the operation of the apparatus Eige. 1141,;11b. ilnttheisimplified:equivalent circuit diagram of..-F ig."=2,ithe input voltage at the terminals ill-is representedbyE rand the-resistance andinductance of the source .byi Rg and Le, respectively. As stated above, the opposed windings of the bridge circuit preferably, have equal numbers .of turns so that the impedance ofeach of'the windings '14 and 17 is represented by theinductance L1 and the. resistance R1,, while that of each of the windingslS, 16 is representedfby'h, R2. It is a well known characteristic of suchabridge circuit that the output voltage-Happeningat*the-terminals 2t 20 is zero Wheneverthewol'tage"dropacross'theWinding 14 is equal to thataeacross.thc winding t1 5-both in -pha'se and magnitude. withzthersymmetrical'construction shown, and-with each OftIhBIlWifldilJESl1i417eiTlGlLlSiVe having the same number ofatum's this accents-when the. armature Z3 is at its center PQSitiOILyQll (F ig. tlgzr). .-Motion of 'the armature '23 to eithcrxsideoflthis center-position develops an'outputvoltage E dependent in pha-se and amplitude-upon the :sense and, magnitude, r espectively,-of the displacement of the armature. This arrangement is preferable for applications in' which the.mechanical efiect to be measuredrrises and fallsfronr an intermediate reference value.

The vector diagram of the voltages of the bridge circuit with the.armature23'displacedfrorn its center position to unbalance the bridge is given, in Fig. '3. The upper vector AB'shows the distribution of the supply voltage Eg ,between the windings"14'arid"I6. "This voltage comprises the voltage AC across winding 14 consisting of the re- ..sistancedrop I1R1 and. the resistance drop I wL While the voltage CB across winding 16 consists of the resistance drop I1R2 and the reactance drop LwL If the impedance of the indicating device 22 or other load connected to the output terminals 27 is so large compared to the impedance of the bridge circuitthat it may be neglected, which is usually the case, then the currents through windings 14, 1'6 are-equal andinay-be representedby Ii as shown. If thephase angles 014 and 01s,.tl1at is, theuratios of inductive reactance to resistance of the windings .14 and 16 are equal, :as'mentioned above, ,point C will lie on the straight line AB.

The lower portion of the vectordiagramof Fig. 3-represents the voltage distribution across the windings 15 and 17. For the sake of clarity, the vector diagrams of the two sides of the bridge circuit have been shown one below the other rather than superimposed, which they actually are. In the lower portionof the diagram, the vectorAD represents the voltage'across winding 15 consisting of the resistancedrop IzRz and the reactance drop I- wL While the vector DB similarly represents the volt age drop across-the winding 17 consisting of the resistance dropslzRirand the reactance drop .l,wL,. ThepointD has been projected on the upper diagram as D, so that the vectorCD' represents the output voltageE of the apparatuszappearing .at the output terminals 27. If the phase angles "014, .016 of-the windings M, 16, respectively, are not eqnal, the 'point C will nolonger lie on the vector AB. Similarly,.if the phase angles 615, we of thewindings 15 and .17, respectively, are unequal, the point D will no longerzlie on the veltor AB. Inthis event the output voltage E will not, except in unusual circumstances, be parallel'to the vector AB so that it willinclucle a component-in quadraturc'to the input voltage. This is undesirableas it makesit impossible to get a-zero output from the :bridge circuit regardless of .the position of the armature 2-3.

The design computations for an electromechanical transducer of thetype described may be considerably simplifiedby the following assumptions, which. it hasbeen established experimentally may be made without limiting theaccuracy of the apparatus beyond that-due to usual manufacturing tolerances when, referring to-Fig. 4, the maximum motion of thearmature 23.at the axis-of the associated poles 10, 11 from its center of symmetry is small compared to the distance c between the armature at.its.centerof-rsymmetry and the pole'face:

1) Theresistances R1 and R2 of the windings are equal to-aresistance of value R.

(2) The'reluctance of the core structure 9-is so small relativ.e:to that of the air gaps that it may be neglected.

.(3.) LThepermeance between-the armature .23 and its associated; pole faces comprises solely the permeancc of the directly opposing areas of the pole faces and the armature.

('4) The armature 23, over its small range of motion defined above, may be considered parallel to thepole faces I01: and 11a.

('5) The impedanceof the indicator device 22 or other loadrcircuit is infinite, or so large relative'to the bridge circuit'i-mpedances that it may be neglected.

('6) All leakage fluxesare suificiently small-that they maybe neglected. Such leakage fluxes ordinarily-do not alterzthe..basic-;relationshipsset forth below but may act as effective magnetic shunts, thereby reducing the sensi tivity of the apparatus.

With the foregoing-assumptions, it can be shown that the output. signal :E .andtheimpedance Z, asseen by the load, rare I represented 1 by v the following relations:

masses where r=efiective radius of the armature 23 from its axis 24 to the point 230. a=angular deflection of armature 23 from its center position.

From Equation 1 it is seen that the output voltage E of the transducer is directly proportional to its angular movement a from its center position.

The design formulae of Equations 1 and 2 may be further simplified if the following assumptions are realized:

(1) The impedance Rg, Lg of the source is so small in comparison to the impedances R1, L1 and R2, L2 that it may be neglected.

(2) The resistances R1, R2 of the transducer windings are small compared to the inductive reactances of the windings wLl and wLz.

With these assumptions, Equation l is simplified as follows:

Since, for small values of a,

Equation 3 may be Written as Thus, under the assumptions and approximations set forth above, the output voltage E, in terms of the input voltage Eg, varies proportionately to the motion x of the armature.

As mentioned above, any leakage fluxes between the poles which do not link the armature 23 affect the sensitivity or scale factor of the apparatus and they also affect the position of the armature for Zero output. This property may be made use of to efiect an adjustment in these characteristics of the apparatus. To this end, a magnetic shunt is disposed adjacent at least one pole of each of the two pairs of poles 10, 11 and 12,13. As shown in Figs. la, lb, there are provided a pair of magnetic shunts 28, 29 individually disposed adjacent the poles 10, 12 associated with the windings 14, 16, respectively, comprising adjacent arms of the bridge circuit connected in series across the input terminals 19, 19. As brought out hereinafter, these shunts are independently adjustable to adjust either the zero position of the armaure 23 or the sensitivity or scale factor of the apparatus, or both. These magnetic shunts 28, 29, as Well as the laminations of the core structure 9, are preferably made of magnetic material of high permeability, such as No. 4750 nickel-iron alloy or high silicon steel, and are effective to divert or shunt a certain amount of the flux from the poles of the windings 14, 16 from the armature.

The shunts 28, 29 are arranged to be moved toward or away from the core structure 9 in a plane normal to it. By moving only one shunt, for example, the shunt 28, the output voltage may be varied. Thus, when the shunt 28 is moved nearer the core structure 9, the inductance of winding 14 is increased, while that of the winding 16 remains the same. Therefore, the voltage drop across winding 14 increases, while that across Winding 16 de creases. Conversely, if the shunt 29 is moved closer to the core structure 9, the inductance of winding 16 is increased and its voltage drop is increased, while that of winding 14 is decreased, and vice versa. Thus, referring to Fig. 3, it is possible to move the point C back and forth along the vector AB, thereby modifying the output voltage E, without any adjustment of the neutral position of 6 the armature 23. At the same time, the distribution of the voltages across windings 15 and 17 is not materially affected so that the point D on vector AB remains stationary. Thus, if the output voltage E of the apparatus is not Zero when the armature 23 is in its initial position as determined by the mechanical arrangements, it is possible to adjust it to zero by adjusting one or the other of the magnetic shunts 28, 29.

On the other hand, if the two magnetic shunts 23, 29 are moved toward or away from the core structure 9 in unison, a greater or lesser amount of fiux is diverted from the armature 23 so that the sensitivity or scale factor of the transducer is reduced or increased, respectively; that is, adjustment of the shunts in unison is effective primarily to modify the calibration characteristic of the transducer, while adjustment of only one of the shunts, or of the two shunts in opposite senses, is effective priman'lyto obtain zero output voltage of the transducer for a desired initial position of the armature 23. The provision of such adjustments by means of the magnetic shunts 28, 29 contributes very considerably to the overall stability of the transducer since it eliminates the necessity of providing mechanical adjustments of the armature 23. Such adjustments are very important because all transducers of a given rating must be designed to provide the same calibration characteristics so that they may be used interchangeably.

In the transducer described above, it has been assumed that it is desired to respond to a mechanical effect or displacement which rises or falls from an intermediate reference value, that is, one which changes in sense or polarity. For some applications it may be desirable to provide a transducer which responds to mechanical effects of only a given sense or polarity from a reference value. Thus, in measuring atmospheric pressure it is customary to measure it from a reference value corresponding to sea level. Since all atmospheric pressures normally measured are less than at sea level, it is desirable in this instance to provide a transducer which responds to a mechanical effect of only a single polarity. Since the response of an electromechanical transducer becomes progressively more non-linear as its deflection range increases, in such cases it is desirable for maximum linearity to make the zero position of the armature one extreme position, for example the position represented by the deflection a in Fig. 4. This can be accomplished in the transducer of the present invention by giving the windings comprising adjacent arms of the bridge circuit unequal numbers of turns and thus unequal values of impedance at the center position of the armature and proportioned to have equal values of impedance to provide substantially zero signal output in the extreme position of the movable armature.

Since the inductance of each of the windings varies as the product of the square of the number of turns and the permeance of its associated air gap, it is possible to make the inductances of the windings equal with the armature 23 in one extreme position by adjusting the turns of the windings to compensate for the difference in the air gap permeance under the faces of the opposed poles, for

example between the armature 23 and the pole face 11a and between the armature 23 and the pole face 10a of Fig. 4. if the inductances are made equal in this fashion and the resistances are also made equal, the bridge circuit will be in balance and the output voltage will be zero. With this arrangement, as contrasted with the symmetrical arrangement first described, there will be an actual flux through the armature in its zero position, that is, its position of zero output signal. Movement of the armature from its extreme position unbalances the bridge in the same manner as that described above and the foregoing equations apply equally to such a modified design, if the armature deflection a be measured from such extreme position.

It can be shown that the ratio between the, turns of the windings 15 and 14 to effect a value of zerooutput voltage when the armature is displaced initially by the Z. amount x. from its. position. of symmetry; as shown 1 in Fig... 4;. injthe .sirnple casewhere leakage flux .is neglected, is given by the following equation:

where N1, Nz=numbers of turns of the=windings14,=15,x

respectively.

Referring. now to- Fig.1 6 there is represented an electro mechanical transducer of commercialnform" incorporating the pick-ofl-device of the invention. Fig. 6 includes a-structurally rigid:. annular housingzmeme ber 30 having a rigid. cylindrical base'rnembers3liadapted. to be rigidly secured to thehousingmemb'er 30'bymeans' of -machine-screws 32,. or equivalent to form a rigid uni-' tary structure; If desired,: a packingvring33 may be interposed betweentthe 'base' member-Bland the housing. to provide a fluid-tight-seal. The cylindrical core st'ruc-- ture 9 is proportioned to be tightly fitted within a second rigid cup-shaped housing member '34 -having an'sannular' flange 34a which may be spun over'the edge of the core The. housing. member34 is then tightly fitted within-:an: annular recess" structure 9' to holdit firmly in position.

30a of member-30, as-by press-fitting, to :form a unitary housing comprising, themembers 30, 31,234.:

The magnetic armature or vane 23 is supported'from the basemernber 31, whereby spurious movements of the armature'relative to the core structure 9 aresub'stantiallyu eliminated. The device of Fig. 6 is adaptedto the measurement of barometric pressure and to this end 'a pressure sensitive element is'provided fon'semi-rigidity butresil ientlysupporting the armature 23 from--the:base"mem' ber 31 and -for rotating it'inresponse to variation'of ambient pressure. Specifically,

the armature 23.

external surface of-thetube, one end may be seale'd off, as indicated at 35a. Onthe other hand, if it is intended to respond to various sources of fluid pressure, the end- 35a may be left open for connection to any desired fluid pressure source.

The magnetic shunts 28, 29 are rigidly secured on posts 36 and 37, respectively, whichv are longitudinally slidable' in bores in the housing member 34. These posts are provided with racks, such as therack 37a of post 37, or equivalent adapted to be operated by a pinion 33011 the end ofan adjustingtoolGQ, the-tool being inserted in transverse bores in the base 34 for engaging the racks of the posts 36 and 37, respectively. The: posts 3oand 37 are mounted in such a-. way as to provide ample friction for retainingthem in their adjusted positions or may be' locked in adjusted'position by set-screws or the'like (not Core structure 9:

Stack-of 13 17' millaminations of.No. 4750 nickeliron; alloy 1 /8 inches diameter, giving stack height of A inch.

Four poles" '10"-13 inc., length-01496 inch; width .250 inchu Spacingc: 0.037 inch:

The:;. apparatus. of

this pressure-sensitive:-v element is'in the form of an elongated, spiral-fluted Bourdon tube 35 having one end rigidly supportedin the base member 31and the other end rigidly supporting- If the Bourdon" tube 35ris intended to respond solely to ambient pressure, acting: on the 1 gram of the: winding connections.

. ence of thG'rPOlE fluxes.

Armature 23:

Length 2r /1 inch; width inch; thickness 0.026

inch. Maximum displacement x=0.00375 inch.

' Windings 14, 17:

2800 turns No. 40 wire.

3100 turns No. 40 wire.

L20.50O henry; R2 385 ohms.

Magnetic. shunts 28, 29:

No. 4750 nickel-ironr-alloy; length %3i11Ch;,.Width' A inch; ,thickness 20026 5 inchi. Range ofwspacing. from core StlllCillIi6z9*0.00=1 inch:

to 0.030 inch.

Temperature compensating: shl1nts--25,"- 26*.

30% nickel, nickel-iron alloy'having-a temperature" coefficient of relative permeability of 0.48" per degree "F;

Cross section-at neck*0.'030"inch' x 0.002 inch.

Input voltage: 15 volts.

The response characteristic of apparatus having the specifications given above is represented in Fig. 5 in which curve A represents variations. inthe output signal of the device in arbitrary units plotted against the displacement of the armature 23 from its zero position, .which is the; position displaced by angle or from its central or neutral position. Curve B represents on an enlarged scale the deviation of the response from;linearity expressed'asa percentage of the signal output for maximum deflection. It is vseen that the percentage error varies within a range of approximately +06% to 0.4%, although this error is :near-the limit' of-accuracyof measurement of the ap paratus. Thus it is seen that .the response of the apparatus-sis almost exactly linear over 'its full operating range.-

In the electro-mechanical transducer described above;- theswindingstof'each of the-pairs 14, 16 and 15; 1 7 are connected in: series across th'e input terminals 19, 19 with such-polarities :that th'e fluxes -r-4, inclusive, are all in the same. directionand: only the diflerence ofthe polefluxes-passes'through the armature 23; In this arrangement, the apparatus-responds' only to pivotal-movement of the armature 23"and translatory movement of the armature: introduces a certain amount of error. Other connectionsmf thewindi'ngs' l l l l, inclusive, and other polarities- 0f connections providetransducers suitable forpivotal movement of the armature or translatory movementot the? armature, or either pivotal ortranslatory movement. Certain ofthese arrangements are shown" schematically-in the remaining figures of the drawings; ThusFig; 7axis aschematic:representation:of the apparatusxof Fig." 1a, while Fig.7cis-a schematic circuit dia- The-windings 14'and l6" are indi'eated' as zbeing adjustable, representing the adjustment of the inductances-ofithese windings by themagnet-icnshunts 28; 29," respectively.. Asstated above, thisarrangement is suitable only for pivotal movement of the armature. Fig; 7b represents another arrangement and connection of windings-for producing the fluxes shownin-Fig: 7asand having similar characteristics, comprising essentially an: interchange of the input and output connecti Onse.

Winding; connections and polarities for transl-atory mover'nentof the armature perpendicular to the pole facesare shown inFigs; 8a.8c, inclusive, in-which similar reference symb0ls-identify corresponding elements; Asshown in Fig,- 8a, the armature carries "only the differ- Windingconnections permitting, either pivotal-or translatory movement of the armature are represented .in Figs. 9a:9e, inclusive. As shownin Fig. 9a,. the. polarity. of. the winding connections is such thatthe armature. carriesithe sumiof the pole fluxes.

When. the windings are connected as shown in either.

of Figs. 9b or 9c an output signal results from pivotal movement of the armature, whereas when the windings are connected as shown in either of Figs. 9d or 9e an output signal results from translatory movement of the armature.

Thus it is seen that the electro-mechanical transducer of the invention has a number of desirable characteris tics not found in such transducers heretofore proposed. The apparatus is relatively insensitive to undesired displacement of the armature. The design of the windings is such as to attain zero output at any desired position of the armature. This feature, with the other constructional features, imparts to the apparatus a response characteristic which is almost exactly linear over its entire operating range. The arrangement of magnetic shunts 28, 29 provides for adjustment of either the zero setting output of the apparatus or the calibration characteristics, or both, without any mechanical adjustment of the armature. The design and proportioning of the windings of the apparatus and their connection in a bridge circuit having like impedances in each arm at neutral zero output position of the armature makes the device relatively insensitive to variations in frequency of the input signal.

While there have been described what are at present considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Certain of the mechanical features of the structure shown in Fig. 6 including the particular means for mounting and adjusting the magnetic shunts, are not claimed herein as they do not form part of the present invention.

We claim:

1. A magnetic pick-off device comprising a circular closed magnetic core structure having two opposed pairs of inwardly extending poles with opposed pole faces, a magnetic armature disposed between said opposed pairs of poles and movable in response to mechanical effect to be represented, a winding disposed on each of said poles, each of said windings comprising an arm of a bridge circuit, and means for adjusting both the scale factor of the pick-off device and the armature position for Zero signal output of the bridge circuit, said means comprising an adjustable magnetic shunt disposed adjacent one of said poles and said core and a second adjustable magnetic shunt disposed adjacent another one of said poles and said core, said shunts when adjusted together being effective to vary the scale factor of the device and when adjusted individually being effective to vary the armature position for zero signal output.

2. A magnetic pick-off device comprising a circular closed magnetic core structure having two opposed pairs of parallel inwardly extending poles with parallel opposed pole faces, a magnetic armature pivoted about the axis of symmetry of said core structure and movable in response to a mechanical effect to be represented, a winding disposed on each of said poles, each of said windings comprising an arm of a bridge circuit, and means for adjusting both the scale factor of the pick-off device and the armature position for zero signal output of the bridge circuit, said means comprising an adjustable magnetic shunt disposed adjacent one of said poles and said core and a second adjustable magnetic shunt disposed. adjacent another one of said poles and said core, said shunts when adjusted together being efective to vary the scale factor of the device and when adjusted individually being eifectiv'e to vary the armature position for zero signal output.

References Cited in the file of this patent UNITED STATES PATENTS 1,608,872 Wallis Nov. 30, 1926 1,988,458 Minorsky Jan. 22, 1935 2,027,140 Alexander Ian. 7, 1936 2,240,184 Hathaway Apr. 29, 1941 2,400,571 Olesen May 21, 1946 2,447,331 Grifith Aug. 17, 1948 2,488,734 Mueller Nov. 22, 1949 2,563,899 Wiancko Aug. 14, 1951 2,564,221 Hornfeck Aug. 14, 1951 2,618,776 Wiancko Nov. 18, 1952 FOREIGN PATENTS 809,389 France Dec. 3, 1936 970,552 France Jan. 5, 1951 

